1. Field of the Invention
The present invention relates to an optical measuring apparatus for measuring characteristics of a test section using light emitted from a light source.
2. Description of the Related Art
Various types of optical measuring systems for measuring characteristic of a test section using light emitted from a light source are known. Optical measurement allows nondestructive testing of semiconductors, disks, living bodies, and the like. For example, optical coherence tomography systems (OCT systems) for obtaining an optical tomographic image of a test section of a living body or the like have already been put into practical use in the medical field or the like. In such systems, the optical tomographic image is obtained by irradiating light on the test section and using the light reflected from or transmitted through the test section.
Various types of OCT systems have been developed. An OTDR (optical time domain reflectometry) that uses an SLD (super-luminescent diode) as the light source is used in eye clinics or the like. In the OTDR described above, low coherence light emitted from the SLD light source is branched into measuring light and reference light by a beam splitter. The reference light is inputted to a reference light mirror which is periodically driven by a PZT and the measuring light is irradiated on the test object. The reference light reflected from the reference light mirror and the measuring light incident on the test object and reflected from a predetermined depth of the test object are combined by the beam splitter and inputted to a single mode fiber. That is, the optical path is formed by a Michelson interferometer. Here, if the optical path length of the reference light and that of the measuring light correspond with each other within the range of the coherence length of the low coherence light, the interference between the reference light and measuring light is observed, and if they do not, the interference is not observed.
Interference fringes may be observed through heterodyne detection of the envelopes of the fringes using a photo diode. The position of the test object relative to the reference light mirror may be measured by periodically scanning the positions in the optical axis direction of the reference light mirror. The tomographic image of the test object may be obtained by periodically scanning the positions in the optical axis direction of the reference light mirror and scanning the test object in the direction orthogonal to the optical axis simultaneously.
The OTDR described above requires a mechanical scanning in the optical axis direction. Recently, however, development work is proceeding with an OFDR (optical frequency domain reflectometry), which is an OCT system that does not require scanning in the optical axis direction. The OFDR also comes in variety of types. For example, an OFDR using broadband spectral interferometry may obtain a tomographic image of test section by detecting interference light from the interferometer as a channeled spectrum in which the interference light is optically broken up into each wavenumber component and analyzing it through Fuorier analysis without performing the scanning in the depth direction as described, for example, in the document entitled “Optical Coherence Tomography” by A. F. Fercher, Journal of Biomedical Optics, Vol. 1, No. 2, pp. 157-173, 1996.
When measuring characteristics of the interior structure of the test section or the like, it is preferable to use light with a wavelength that matches with the light transmission characteristics of the test section, that is, the wavelength having a highest optical transmittance for the test section. Further, it is preferable that different optical measuring methods use light with different wavelengths appropriate for the respective methods.
In the conventional light source that uses a laser, an SLD or the like, however, it is difficult to set the oscillation wavelength arbitrarily because the wavelength of the light emitted from the light source is determined by the composition of the gain medium. Thus, the optical measuring system using such light source has a problem that it is unable to perform measurement using light with an appropriate wavelength according to the optical transmittance, measuring method employed, or the like.
The present invention has been developed in view of the circumstances described above, and it is an object of the present invention to provide an optical measuring apparatus that allows measurement using light with an appropriate emission wavelength according to the optical transmittance of a test section, measuring method employed, or the like.